The River Trail Greenway runs along the Arkansas River all the way from Marvin Park to Franz Lake, by way of Sand Lake. This 1.7-mile trail is all relatively flat, wide, and consists of crushed gravel. For mountain biking, this is about as easy as it gets, but if you're looking for a chill afternoon cruise with a non-biking friend or significant other with great views of the river, this is a great choice!
Marvin Park or at CR 175
http://www.singletracks.com/item.php?c=1&i=8518